It was previously proposed in the copending United States patent application Ser. No. 08/122,615 filed Sep. 16, 1993, U.S. Pat. No. 5,528,497; Ser. No. 08/161,139 filed Dec. 2, 1993, U.S. Pat. No. 5,448,481; Ser. No. 08/218,134 filed Mar. 25, 1994, U.S. Pat. No. 5,448,482; Ser. No. 08/218,116 filed Mar. 25, 1994, U.S. Pat. No. 5,481,457; and Ser. No. 08/218,135 filed Mar. 25, 1994 to compute a deviation of the actual yaw rate of the vehicle from a standard yaw rate determined according to the steering angle of the steering wheel and the vehicle speed, and carry out a feedback control of the rear wheel steering angle for the purpose of optimizing the dynamic response of the vehicle, and removing the adverse influences of the external disturbances and the changes in the running conditions of the vehicle on the behavior of the vehicle. The contents of these copending applications are incorporated in the present application by reference.
When the limit of the road gripping capability of a tire is exceeded on a road surface having a low frictional coefficient such as a gravel road and an icy road, the cornering force is dramatically reduced. Also, in a front and rear wheel steering vehicle, if abrupt and large-angle steering maneuvers are continued on a winding road, the capability of the rear wheel steering actuator to respond to the control command could be lost clue to the shortage of power of the actuator for steering the rear wheels. In such situations, according to the previously proposed steering systems, some discrepancies (which may be sensed as a phase difference) may be produced between the actual rear wheel steering angle produced by the yaw rate feedback control, and the anticipation or the expectation of the vehicle operator. This is not desirable because the vehicle operator should be the ultimate controller of the vehicle, and should be able to accurately grasp the condition of the vehicle at all times to the end of comfortably and safely operating the vehicle.